Efficient collection and concentration of radiant energy is useful in a number of applications and is of particular value for devices that convert solar energy to electrical energy. Concentrator solar cells make it possible to obtain a significant amount of the sun's energy and concentrate that energy as heat or for generation of direct current from a photovoltaic receiver.
Large-scale light concentrators for obtaining solar energy typically include a set of opposed, curved mirrors with a Cassegrain arrangement as an optical system for concentrating light onto a receiver that is positioned at a focal point. As just a few examples employing the Cassegrain model, U.S. Pat. No. 5,979,438 entitled “Sunlight Collecting System” to Nakamura and U.S. Pat. No. 5,005,958 entitled “High Flux Solar Energy Transformation” to Winston et al. both describe large-scale solar energy systems using sets of opposed primary and secondary mirrors. As a more recent development for providing more compact collection apparatus, planar concentrators have been introduced, such as that described in the article entitled “Planar Concentrators Near the Etendue Limit” by Roland Winston and Jeffrey M. Gordon in Optics Letters, Vol. 30 no. 19, pp. 2617-2619. Planar concentrators similarly employ primary and secondary curved mirrors with a Cassegrain arrangement, separated by a dielectric optical material, for providing high light flux concentration.
FIG. 1A shows the basic Cassegrain arrangement for light collection. A photovoltaic apparatus 10 with an optical axis O has a parabolic primary mirror 12 and a secondary mirror 14 located near the focal point of primary mirror 12. A receiver 16 is then placed at the focal point of this optical system, at the vertex of primary mirror 12. A recognized problem with this architecture, a problem inherent to the Cassegrain model, is that secondary mirror 14 presents an obstruction to on-axis light, so that a portion of the light, nominally as much as about 10%, does not reach primary mirror 12, reducing the overall light-gathering capability of photovoltaic apparatus 10. This obscuration can be especially large if the concentrator is cylindrical instead of rotationally symmetric. Placement of receiver 16 at the vertex of primary mirror 12, in the path of the obstruction presented by secondary mirror 14, helps somewhat to mitigate losses caused by the obstruction. However, with a cylindrical optical configuration, little or none of this obstruction loss is gained back by making dimensional adjustments, since the size of the obstruction scales upwards proportionally with any increased size in primary mirror 12 diameter. This means that enlarging the diameter of the larger mirror does not appreciably change the inherent loss caused by the obstruction from the smaller mirror.
Some types of solar energy systems operate by converting light energy to heat. In various types of flat plate collectors and solar concentrators, concentrated sunlight heats a fluid traveling through the solar cell to high temperatures for power generation. An alternative type of solar conversion mechanism, more adaptable for use in thin panels and more compact devices, uses photovoltaic (PV) materials to convert sunlight directly into electrical energy. Photovoltaic materials may be formed from various types of silicon and other semiconductor materials and are manufactured using semiconductor fabrication techniques and provided by a number of manufacturers, such as Emcore Photovoltaics, Albuquerque, N.M., for example. While silicon is less expensive, higher performance photovoltaic materials are alloys made from elements such as aluminum, gallium, and indium, along with elements such as nitrogen and arsenic.
As is well known, sunlight is highly polychromatic, containing broadly distributed spectral content, ranging from ultraviolet (UV), through visible, and infrared (IR) wavelengths, each wavelength having an associated energy level, typically expressed in terms of electron-volts (eV). Not surprisingly, due to differing band-gap characteristics between semi-conductor materials, the response of any one particular photovoltaic material depends upon the incident wavelength. Photons having an energy level below the band gap of a material slip through. For example, red light photons (nominally around 1.9 eV) are not absorbed by high band-gap semiconductors. Meanwhile, photons having an energy level higher than the band gap for a material are absorbed. For example, excess energy from violet light photons (nominally around 3 eV) is wasted as heat in a low band-gap semiconductor.
One strategy for obtaining higher efficiencies from photovoltaic materials is to form a stacked photovoltaic cell, also sometimes termed a multifunction photovoltaic device. These devices are formed by stacking multiple photovoltaic cells on top of each other. With such a design, each successive photovoltaic cell in the stack, with respect to the incident light source, has a lower band-gap energy. In a simple stacked photovoltaic device, for example, an upper photovoltaic cell, consisting of gallium arsenide (GaAs), captures the higher energy of blue light. A second cell, of gallium antimonide (GaSb), converts the lower energy infrared light into electricity. One example of a stacked photovoltaic device is given in U.S. Pat. No. 6,835,888 entitled “Stacked Photovoltaic Device” to Sano et al.
While stacked photovoltaics can provide some measure of improvement in overall efficiency, these multilayered devices can be costly to fabricate. There can also be restrictions on the types of materials that can be stacked together atop each other, making it difficult for such an approach to prove economical for a broad range of applications. Another approach is to separate the light according to wavelength into two or more spectral portions, and to concentrate each portion onto an appropriate photovoltaic receiver device, with two or more photovoltaic receivers arranged side by side. With this approach, photovoltaic device fabrication is simpler and less costly, and a wider variety of semiconductors can be considered for use. This type of solution requires supporting optics for both separating light into suitable spectral components and concentrating each spectral component onto its corresponding photovoltaic surface.
One proposed solution for simultaneously separating and concentrating light at sufficient intensity is described in a paper entitled “New Cassegrainian PV Module using Dichroic Secondary and Multijunction Solar Cells” presented at an International Conference on Solar Concentration for the Generation of Electricity or Hydrogen in May, 2005 by L. Fraas, J. Avery, H. Huang, and E. Shifman. In the module described in this article and schematically represented in FIG. 1B, curved primary mirror 12 collects light and directs this light toward a dichroic hyperbolic secondary mirror 14, near the focal plane of the primary mirror. IR light is concentrated at a first photovoltaic receiver 16 near the focal point of the primary mirror. The secondary mirror redirects near-visible light to a second photovoltaic receiver 18 positioned near the vertex of the primary mirror. In this way, each photovoltaic receiver 16 and 18 obtains the light energy for which it is optimized, increasing the overall efficiency of the solar cell system.
While the approach shown in the Fraas paper advantageously provides spectral separation and concentrates light using the same set of optical components, there are some significant limitations to the solution that it presents. A first problem relates to the overall losses due to obstruction of the aperture, as were noted earlier. As another problem, the apparatus described by Fraas et al. has a limited field of view of the sky because it has a high concentration in each axis due to its rotational symmetry. Yet another drawback relates to the wide bandwidths of visible light provided to a single photovoltaic receiver. With many types of photovoltaic materials commonly used for visible light, an appreciable amount of the light energy would still be wasted using such an approach, possibly resulting in excessive heat.
Dichroic surfaces, such as are used for the hyperbolic mirror in the solution proposed in the Fraas paper, provide spectral separation of light using interference effects obtained from coatings formed from multiple overlaid layers having different indices of refraction and other characteristics. In operation, dichroic coatings reflect and transmit light as a function of incident angle and wavelength. As the incident angle varies, the wavelength of light that is transmitted or reflected by a dichroic surface also changes. Where a dichroic coating is used with incident light at angles beyond about +/−20 degrees from normal, undesirable spectral effects can occur, so that spectral separation of light, due to variations in the angles of incidence, is compromised at such higher angles.
There have been a number of light collector solutions employing dichroic surfaces for spectral splitting. For example, in an article entitled “Spectral Beam Splitting Technology for Increased Conversion Efficiency in Solar Concentrating Systems: A Review”, available online at www.sciencedirect.com, authors A. G. Imenes, and D. R. Mills provide a survey of solar collection systems, including some using dichroic surfaces. For example, the description of a tower reflector (FIG. 24 in the Imenes and Mills article) shows one proposed solution that employs a curved dichroic beamsplitter as part of the optics collection system. High incident angles of some portion of the light on this surface could render such a solution as less than satisfactory with respect to light efficiency. Similarly, U.S. Pat. No. 4,700,013 entitled “Hybrid Solar Energy Generating System” to Soule describes the use of a dichroic surface as a selective heat mirror. However, as noted in the Imenes article cited above, the approach shown in the Soule '013 patent exhibits substantial optical losses. Some of these losses relate to the high incident angles of light directed to the selective heat mirror that is used.
There are inherent problems with dichroic surface shape and placement for light focused from a parabolic mirror. A flat dichroic surface positioned near the focal region of a parabolic reflector would exhibit poor separation performance for many designs, constraining the dimensions of a light collection system. A properly curved dichroic surface, such as a hyperbolic surface, can be positioned at or near the focal region, but obstructs some portion of the available light, as noted earlier.
FIG. 1C shows a simplified version of a conventional solution that has been proposed in numerous embodiments for spectral separation in a radiant energy collection apparatus using a flat dichroic beamsplitter 20. Incident light is concentrated by a lens 22 and directed to beamsplitter 20, oriented at 45 degrees, which reflects one portion of the spectral band to first photovoltaic receiver 16 and transmits another portion of the spectral band to second photovoltaic receiver 18. This general type of solution is described as a “lateral optical system” in a paper entitled “50% Efficient Solar Cell Architectures and Designs” by Barnett et al. presented at 2006 IEEE 4th World Conference on Photovoltaic Energy-Conversion. FIGS. 19, 23, and 24 in the Inenes et al. article cited earlier show some of the alternative configurations based on this general solution.
The lateral optical type of system exhibits good optical transmission levels but has relatively low efficiency. This is at least partly due to a significant amount of spectral contamination resulting from the relatively high incident angles of light on the dichroic surface of beamsplitter 20. Dichroic coatings reflect and transmit light as a function of incident angle and wavelength. As the incident angle varies, the wavelength of light that is transmitted or reflected also changes. Thus, the spectral content of the light reflected from beamsplitter 20 varies over the surface of receivers 16 and 18, reducing the efficiency of energy conversion.
Against obstacles such as poor dichroic surface response, conventional approaches have provided only a limited number of solutions for achieving, at the same time, both good spectral separation and efficient light flux concentration of each spectral component. The Cassegrain model of FIGS. 1A and 1B can be optimized, but always presents an obstruction near the focal point of the primary mirror, and is thus inherently disadvantaged. Solutions that employ dichroic separation perform best where incident light angles on the dichroic surface are low with respect to normal; however, as exemplified in FIG. 1C, many proposed designs do not appear to give enough consideration to these spectral separation characteristics, resulting in poor separation or misdirected light and reduced efficiency.
Thus, it is recognized that there is a need for a photovoltaic cell that provides improved spectral separation and light concentration, that can be easily scaled for use in a thin panel design, that can be readily manufactured, and that offers increased efficiency over conventional photovoltaic solutions.